Hello supasizefries,
The most common way to do the 2D build with the 2/3A cells is to have a welded pack assembled and shrink-wrapped. CPF member Luxluthor used to make many packs like that, I'm not sure if he is still doing it or not though.
An "adapter" for those cells isn't entirely out of the question, but I don't think it could be made to fit into a 2D mag, 2.5D would be about the shortest length possible with an adapter involved. At this time, I have not designed an adapter for "A" cells in this configuration, but do have the materials and tooling to make such a product without much trouble I believe. Keep in mind the host has to be bored to fit 3 "A" diameter cells abreast.
-Eric